iterator

核心变量：

cursor：指向下一个将要被访问的元素的索引号
lastRet：指向刚被访问的元素的索引号
expectedModCount：通过与ModCount比较，判断是不是进行了iterator内部remove的增删操作

核心方法：

hasNext（）：比较cursor与size（）
next（）：返回下一个元素
remove：删除集合中的元素，并更新expectedModCount和ModCount
checkForCoModComodification（）：检测expectedModCount和ModCount是否相等

Iterable接口与Iterator接口

Iterable接口只有一个iterator（）方法，用于返回实现了Iterator接口的类
Iterator接口规定了操作集合元素的方法

hasNext（）
next（）
remove（）

源码：来自AbstractList<E>的Itr类，这个类尤iterator（）方法返回

private class Itr implements Iterator<E> {
/**
* Index of element to be returned by subsequent call to next.
*/
int cursor = 0;

/**
* Index of element returned by most recent call to next or
* previous.  Reset to -1 if this element is deleted by a call
* to remove.
*/
int lastRet = -1;

/**
* The modCount value that the iterator believes that the backing
* List should have.  If this expectation is violated, the iterator
* has detected concurrent modification.
*/
int expectedModCount = modCount;

public boolean hasNext() {
return cursor != size();
}

public E next() {
checkForComodification();
try {
int i = cursor;
E next = get(i);
lastRet = i;
cursor = i + 1;
return next;
} catch (IndexOutOfBoundsException e) {
checkForComodification();
throw new NoSuchElementException();
}
}

public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();

try {
AbstractList.this.remove(lastRet);
if (lastRet < cursor)
cursor--;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException e) {
throw new ConcurrentModificationException();
}
}

final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}